May 6  -- Swine flu killed its first U.S. resident, a 33-year-old school teacher from Texas, and new infections in Europe brought the World Health Organization to the verge of declaring a pandemic.

Countries that haven't seen cases should prepare for outbreaks, Richard Besser, acting director of the U.S. Centers for Disease Control and Prevention, said yesterday. He also reversed U.S. school-closure recommendations that shut 468,000 students out of classes yesterday, saying schools should reopen and sick children should stay home.

Disease trackers are monitoring 57 cases in Spain and 27 in the U.K. to determine whether there's evidence the virus has established itself outside the U.S., Canada and Mexico, the three countries struck first and hardest. Such a finding would prompt the WHO to declare a pandemic, the first since 1968, the agency said.

"We continue to see an increase in cases reported by countries," Keiji Fukuda, assistant director-general of health, security and environment, told reporters in Geneva yesterday. "It is clear that we are seeing severe illness in at least a couple of countries."

School Reversal

In the U.S., the CDC said Mexico's initial reports that the virus was disproportionately attacking and killing young people led to the agency's advice that schools close if they suspected anyone were sick with swine flu. Yesterday's reversal was based on research showing the virus causes less severe symptoms than originally feared, Besser said. Also, the virus has already rooted itself in communities across the country, making containment impossible, he said.

Swine flu, also known as H1N1, has been confirmed by laboratory tests in 1,490 patients in 21 countries. Mexico has reported 942 cases, including 29 deaths, while the U.S. has 403 cases and two deaths. Canada has 140 cases.

Those numbers may represent a fraction of the total cases, as more illnesses are reported and health labs continue to test thousands of samples taken early in the outbreak. Before this week, only the CDC lab in Atlanta could definitively identify cases of swine flu in the U.S. Test kits were delivered yesterday to laboratories in all 50 states.

Younger Patients

Data so far suggest that the virus affects youth more than seasonal influenza, and that younger patients are entering hospitals, Besser said. Few with swine flu are older than 60, and the median age is 16. It's possible that older people have greater immunity or that younger people are spreading the disease on spring break vacation trips to Mexico, he said.

Swine flu is suspected in 44 U.S. states, and the infected probably number in the thousands, health officials said. Even if symptoms are mild, the ease with which the new virus spreads makes it a threat, according to the CDC.

Both U.S. deaths have been in Texas. The first was a 22- month-old infant from Mexico taken to a Houston hospital where he died April 28.

The 33-year-old school teacher from the area of Harlingen, Texas, died yesterday after first seeking treatment for a chronic respiratory condition on April 14, said Leo Lopez, an epidemiologist at the Cameron County Department of Health and Human Services in Texas. The teacher returned to the hospital on April 19 with more severe symptoms, he said.

Exacerbated by Flu

"She already had a respiratory condition -- it was exacerbated by the flu," Lopez said in a telephone interview yesterday. He said the woman hadn't traveled recently and the flu was probably "home-grown," contracted in the U.S. Cameron County is located in south Texas along the U.S. border with Mexico.

The three main seasonal flu strains -- H3N2, another form of H1N1, and type B -- cause 250,000 to 500,000 deaths a year globally, according to the WHO. The new flu's symptoms are similar: aches, coughing, and fever.

Mexican Health Minister Jose Cordova said his country is seeing fewer cases of swine flu, although it's too early to say the outbreak is over.

"In the past few days we've seen a big" decline in new cases, Cordova said yesterday in an interview. "We can't say that it's under control, because ‘under control' is when you don't have any new cases."

Open for Business

Restaurants in Mexico City and businesses in most areas of the country are allowed to reopen today and universities and high schools can resume tomorrow. Government offices that had been shut by the flu outbreak also will open today as Mexico begins to return to normalcy. Other schools will start May 11.

In addition to the three North American countries, WHO has confirmed swine flu in Austria, Colombia, Costa Rica, Denmark, El Salvador, France, Germany, China (Hong Kong), Ireland, Israel, Italy, the Netherlands, New Zealand, Portugal, South Korea, Spain, Switzerland and the U.K.

The virulence of the swine flu may reveal itself when the Southern Hemisphere faces its influenza season beginning this month through September, Fukuda said. Scientists will watch the virus to see whether it becomes the dominant flu strain or mutates into a deadlier illness.

In Mexico, the onset of warmer temperatures may be helping quell the flu's spread, said Miguel Angel Lezana, director of the National Center for Epidemiology Control. Swine flu was spreading at about the same rate as seasonal flu, with each new patient infecting, on average, about 1.5 people, said Lezana. The number is an early estimate and may change, he said.

Efficient Approach

While there is no vaccine currently available to fight this flu, shots are the most efficient and cheapest way to stop a pandemic, said Ira Longini, a University of Washington statistician who advises the U.S. government on flu. Closing schools costs about 20 times more, mainly because of lost work hours and services, he said.

The Fort Worth Independent School District in Texas, the 17th largest in the U.S., closed its 145 schools on April 29, based on the recommendation of the local public health officials, Clint Bond, a spokesman for the district, said yesterday in a telephone interview.

School district office staff members will return to work today and teachers tomorrow, the district said in a statement. Classes will resume May 8, according to a statement on the district's Web site. The district of 80,000 students has four cases of swine flu confirmed by the CDC and a further 22 suspected cases, Bond said.

‘Agonizing Decision'

"It was an agonizing decision" to close the schools, Bond said. "It has been incredibly inconvenient for our parents, for our staff and for our community, but we thought it was the right decision to make based on the best information available."

The U.S. is hastening production of its annual flu shots based on strains identified before the H1N1 outbreak, said Kathleen Sebelius, who was confirmed as the U.S. Health and Human Services secretary last week. That will make capacity available if vaccines are needed for swine flu, she said.

Sanofi-Aventis SA of Paris, Baxter International Inc. of Deerfield, Illinois, and GlaxoSmithKline Plc of London, are talking with world health authorities about producing shots, the agency said.

Authorities advised hand washing, hygiene and staying home if sick as the most effective ways to control the outbreak. The WHO and CDC said closing borders or killing animals are costly steps that wouldn't slow the spread of flu.